Any ophthalmic lens intended to be held in a frame involves a prescription. The ophthalmic prescription can include a positive or negative power prescription as well as an astigmatism prescription. These prescriptions correspond to corrections enabling the wearer of the lenses to correct defects of his vision. A lens is fitted in the frame in accordance with the prescription and the position of the wearer's eyes relative to the frame.
For presbyopic wearers, the value of the power correction is different for far vision and near vision, due to the difficulties of accommodation in near vision. The prescription thus comprises a far-vision power value and an addition (or power progression) representing the power increment between far vision and near vision; this comes down to a far-vision power prescription and a near-vision power prescription. Lenses suitable for presbyopic wearers are progressive multifocal lenses; these lenses are described for example in FR-A-2 699 294, U.S. Pat. No. 5,270,745 or U.S. Pat. No. 5,272,495, FR-A-2 683 642, FR-A-2 699 294 or also FR-A-2 704 327.
Progressive multifocal ophthalmic lenses include a far-vision zone, a near-vision zone, an intermediate-vision zone, a principal progression meridian crossing these three zones. They are generally determined by optimization, based on a certain number of constraints imposed on the different characteristics of the lens. Most lenses marketed are all-purpose lenses, in that they are adapted to the different needs of the wearers at the time.
A progressive multifocal lens can be defined by geometric characteristics on at least one of its aspherical surfaces. In order to characterize an aspherical surface, the parameters constituted by the minimum and maximum curvatures at each point are conventionally used, or more commonly their half-sum and their difference. This half-sum and this difference multiplied by a factor n−1, n being the refractive index of the lens material, are called mean sphere and cylinder.
Moreover, a progressive multifocal lens can also be defined by optical characteristics taking into account the situation of the wearer of the lenses. In fact, the laws of the optics of ray tracings provide that optical defects appear when the rays deviate from the central axis of any lens. Conventionally, the aberrations known as power defects and astigmatism defects are considered. These optical aberrations can be generically called obliquity defects of rays.
The obliquity defects of rays have already been clearly identified in the prior art and improvements have been proposed. For example, the document WO-A-98 12590 describes a method for determination by optimization of a set of progressive multifocal ophthalmic lenses. This document proposes defining the set of lenses by considering the optical characteristics of the lenses and in particular the wearer power and oblique astigmatism, under wearing conditions. The lens is optimized by ray tracing, using an ergorama associating a target object point with each direction of viewing under wearing conditions.
EP-A-0 990 939 also proposes to determine a lens by optimization taking into account the optical characteristics instead of the surface characteristics of the lens. For this purpose the characteristics of an average wearer are considered, in particular as regards the position of the lens in front of the wearer's eye in terms of curving contour, pantoscopic angle and lens-eye distance.
It has been found that each wearer has a different head-eye behaviour. Recently, it has therefore been sought to personalize progressive ophthalmic lenses in order to best satisfy the needs of each wearer.
The applicant markets, under the trade mark VARILUX IPSEO® a range of progressive lenses, which are defined as a function of the wearer's head-eye behaviour. This definition is based on the fact that any wearer, in order to look at different points at a given height in the object space, can move either his head, or his eyes and that the viewing strategy of a wearer is based on a combination of head and eye movements. The wearer's viewing strategy influences the perceived width of the fields on the lens. Thus, the more the wearer's lateral vision strategy involves a movement of the head, the narrower is the zone of the lens scanned by the wearer's vision. If the wearer moved only his head in order to look at different points at a given height of the object space, his vision would still pass through the same point of the lens. The product VARILUX IPSEO® therefore proposes different lenses, for the same ametropia-addition pair, as a function of the wearer's lateral vision strategy.
It has also been found that the size and the shape of the frame modifies the wearer's lens-eye behaviour. Therefore it has also been sought to optimize the progressive ophthalmic lens for the type of frame chosen.
For example, the patent U.S. Pat. No. 6,199,983 proposes to personalize a progressive lens as a function of the “lifestyle” of the wearer, for example taking into account the shape of the frame.
It is also proposed in the document U.S. Pat. No. 5,444,503 to take into account the shape of the frame in order to distribute the prismatic effects to the left and to the right of the lens in order to obtain an acceptable “thickness-weight” ratio and in order to disperse the aberrations towards the parts of the lens intended to be trimmed during cutting out.
Documents U.S. Pat. No. 6,655,802 and US-A-2004/0169297 propose to optimize a progressive lens as a function of the cornea-vertex distance measured for a given frame in order to determine an optimal progression length.
Nikon® markets under the trade mark Seemax® a unifocal lens optimized as a function of the size and the shape of the frame.
None of the known solutions makes it possible to optimize the lens in all of the wearer's vision field as a function of the frame chosen.
A need still exists therefore for a lens which better satisfies the specific needs of each individual wearer.